A variable capacitance element is an important component used in, for example, electrical circuits including variable frequency oscillators, tuned amplifiers, phase shifters, impedance matching circuits, and the like. In recent years, there has been an increase in the mounting of variable capacitance elements to mobile devices. Variable capacitance elements manufactured using MEMS technology have the advantage of being able to increase the Q value due to having a low loss, compared with varactor diodes that have mainly been used heretofore. For this reason, development of these variable capacitance elements is being pushed ahead.
Variable capacitance elements are commonly configured to change capacitance by changing the distance between two opposing electrodes (e.g., see Patent Document 1). FIGS. 1A and 1B depict the configuration of a conventional variable capacitance element. A fixed electrode 43 is provided on a substrate 41, and a movable electrode 45 is supported in a position opposing the fixed electrode 43. The movable electrode 45 is movable with respect to the fixed electrode 43 because of having elasticity. The distance between the movable electrode 45 and the fixed electrode 43 changes as a result of electrostatic attraction produced by applying a voltage between the fixed electrode 43 and the movable electrode 45. Electrostatic capacitance thereby changes. Also, a dielectric layer 49 is installed between the fixed electrode 43 and the movable electrode 45 in order to prevent short circuits resulting from contact between the electrodes.
With digital variable capacitance elements, the capacitance formed is minimized in a state where the fixed electrode 43 and the movable electrode 45 are separated (FIG. 1A). The voltage (i.e., driving voltage) between the fixed electrode 43 and the movable electrode 45 at this time is given as Voff. Also, the capacitance is maximized in a state where the fixed electrode 43 and the movable electrode 45 are in contact via the dielectric layer 49 (FIG. 1B). The driving voltage at this time is given as Von. Digital variable capacitance elements are used in these two states, that is, the state where the driving voltage is Von and the state where the driving voltage is Voff.
FIG. 1C is a graph depicting the relation between driving voltage (horizontal axis) and electrostatic capacitance (vertical axis) in a variable capacitance element. Electrostatic capacitance increases sharply at a given voltage when the driving voltage is increased, and becomes constant (maximum capacitance) thereafter, and subsequently when the driving voltage is reduced, electrostatic capacitance decreases sharply at a given voltage and then becomes constant (minimum capacitance).
Patent Document 1: Japanese Laid-open Patent Publication No. 2006-261480.
For example, in the case of manufacturing an impedance matching circuit in which a variable capacitance is connected in parallel to a signal line connecting an input terminal In and output terminal Out such as depicted in FIG. 2, the variable capacitance element is formed on a line connecting the signal line and ground. That is, a line is extracted from the signal line, and the variable capacitance element is formed on the extracted line.
In this way, the distance between the signal line and ground increases as a result of the variable capacitance element being inserted. This leads to an increase in device size.